DE4303335B4 - Ferroelectric liquid crystal mixtures with short ferroelectric pitch - Google Patents

Abstract

A ferroelectric liquid crystal material comprising a chiral dopant and a host compound, the ferroelectric liquid crystal composition having a ferroelectric phase and a chiral nematic phase at temperatures above the ferroelectric phase, characterized in that the pitch of the natural helix in the nematic phase is Temperatures of at least about 1 ° C to 2 ° C above the N ^* transition point at least about 4 times greater than the pitch of the natural helix in the ferroelectric phase at about room temperature and the chiral dopant a chiral non-racemic compound of the formula RAZ ₁ -BOM is in the
A and B independently of one another are 1,4-phenylene, 1,4-phenylene in which one or two of the ring carbon atoms are replaced by nitrogen atoms, or 1,4-cyclohexylene,
Z _{1 is} a single bond, an O atom, a -CO-O- or an -O-CO- group,
R is a group of 1 to 20 carbon atoms in which one or more non-adjacent carbon atoms are replaced by a double bond, an O atom, an S atom or an Si (CH ₃ ) ₂ group ...

Description

Territory of invention

The The present invention relates to ferroelectric liquid crystals (FLCs) Chiral non-racemic compositions of the present invention Invention show a helix with a short pitch in the ferroelectric phase and a long pitch in the chiral nematic Phase.

Background of the invention

Lagerwall and Clark described the effect of a surface-stabilized ferroelectric Liquid crystal [surface-stabilized ferroelectric liquid-crystal (SSFLC)] and its application to electro-optical Shutter and Display Devices (U.S. Patents 4,367,924 and 4,563 059). In SSFLC cells, the FLC becomes between transparent electrodes aligned in the so-called "bookshelf" arrangement, in which the smectic Layers are substantially perpendicular to the electrodes and the long axis of the FLC molecules parallel to the electrodes runs. In this configuration, the one in the ferroelectric phase typically formed natural Helix through surface interactions suppressed in the cell. The oppression the helix results in a bistable cell in which the optical Axis of the cell by changing the Sign of the applied driver voltage in the plane of the electrodes rotated by 2Θ can be, in which Θ the Tilt angle is. The angle of inclination is inherent in the FLC Property. To suppress the helix, the cell thickness (d) must be comparable to the size of the pitch or smaller than the latter. So should for applications in the visible; where cell thicknesses of 0.5 to 6 microns most useful are (assuming a birefringence of 0.15 to 0.3), the pitch the natural one Helix of the SSFLC in the FLC longer than 0.5 to 10 μm be.

Electro-optic effects in FLC cells in which the helix in the smectic C ^* phase is not suppressed by surface stabilization have also been described. The ferroelectric effect of a distorted helix [Distorted Helix Ferroelectric (DHF) effect] described, for example, in Ostovski et al., Advances in Liquid Crystal Research and Applications, Oxford / Budapest (1980), page 469, and Funfschilling and Schadt (1989). , J. Appl. Phys. 66 (8): 3877-3882 is observed in FLCs aligned between electrode plates where the pitch of the natural helix in the smectic C ^* phase (or another chiral tilted smectic ferroelectric phase) is sufficient is short, ie shorter than the thickness (d) of the FLC cell, so that the helix is not suppressed. DHFLC electro-optical devices have an FLC aligned between electrode plates. Most typically, the FLC is planar aligned and is in the "bookshelf" geometry. A driving voltage is applied to the electrodes to create an electric field across the FLC layer. Unlike surface-stabilized FLC devices, the natural helix of the aligned chiral smectic phase is present in the targeted FLC material in the DHF effect device. The helix forms parallel to the plates and perpendicular to the smectic layers, as in 1 is explained. The size of the pitch of the helix is the distance along the axis of the helix for a full turn of the helix, and the sign of the pitch (+ or -) indicates the direction of twist of the helix. The term "short" pitch, which may be a positive or negative value, is associated with shorter axial distances for a full turn of the helix. The term "pitch" as used herein refers to the magnitude of the pitch; the terms "sign of the pitch" or "twist" refer to the direction of the twist.

SSFLC and DHF cells can be operated in the mode of reflection, in which a the electrode plates is reflective (see, for example, US Pat. No. 4,799,776).

When the magnitude of the C ^* He1ix pitch is comparable to the wavelength of visible light, a striped pattern appears in the apparatus, and in effect, a diffraction grating is formed. If the size of the pitch is less than the wavelength of the light (and preferably less than ½ λ of the light), the light diffraction is minimized and the apparent refractive index of the FLC is the average over many helix director orientations, as in FIG 1 is shown. In the field-free state with an applied zero electric field and without surface stabilization, the C ^* helix is in its natural state. The molecular director n ^ forms an angle, Θ, with the normal of the layer. In the field-free state (E = 0), averaging occurs due to the presence of the helix, and the apparent optical axis of the DHFLC coincides with the helical axis, as in FIG 1 is shown.

When the voltage applied across the FLC layer is above a certain critical value E _c , the helix is completely de-aired and forms two distinct optical states, as in a SS FLC device. The application of a voltage below E _c deforms the helix and produces an effective rotation of the optical axis of the DHFLC. The orientation of the optical axis of the DHFLC layer may be varied in a contingent manner in proportion to the applied electric field to change the optical anisotropy of the FLC. DHF cells exhibit a rotation of their optical axis, which depends on the magnitude of the applied electric field, and also show a change in apparent birefringence (Δn) as a function of the magnitude of the applied electric field.

The maximum field-induced angle of rotation of the optical axis of the DHFLC is Θ, the angle of inclination of the material. A maximum field induced optical axis rotation of 20 can be obtained by applying a +/- voltage step, +/- E _max , where E _{max is} the minimum required voltage to achieve a rotation of Θ, and Amount of E _{max is} less than E _c .

DHF-effect cells typically show due to the above-noted average formation a significantly lower apparent refractive index than SSFLC cells. So are for a desired one optical delay DHF cells typically thicker than comparatively SSFLC cells. Birefringence for DHFLC cells typically ranges from about 0.06 to about 0.13, about that of SSFLC cells. As a result, DHFLC waveplates are typically thicker as comparable SSFLC wave plates. Materials with high birefringence are useful in DHF applications to minimize cell thickness.

umgekehrt proportional.E _c is the spontaneous polarization of the FLC and the pitch of the ferroelectric phase according to the relationship

inversely proportional.

worin γ die Orientierungs-Viskosität ist und Θ der Neigungswinkel ist. Die Erhöhung von P_s erniedrigt die Schwellenspannung, erhöht jedoch nicht die Geschwindigkeit, während eine Verkürzung der Ganghöhe sowohl die Geschwindigkeit als auch E_c erhöht. Dadurch, daß sowohl P_s erhöht wird als auch p erniedrigt wird, kann die Antwortgeschwindigkeit signifikant gesteigert werden, während eine niedrige Schwellenspannung erhalten bleibt. Auch eine Erniedrigung der Viskosität verbessert die Ansprechzeit.Thus, the higher the spontaneous polarization and the longer the pitch, the lower the voltage required to control the effect. The response time (τ) for the DHFLC cell is defined as

where γ is the orientation viscosity and Θ is the tilt angle. The increase in P _s lowers the threshold voltage, but does not increase the speed, while shortening the pitch increases both the speed and E _c . By both increasing and decreasing P _s , the response speed can be significantly increased while maintaining a low threshold voltage. Also lowering the viscosity improves the response time.

größer als 0,1 und vorzugsweise größer als 0,45 ist. Diese Forderung erlegt den Zelldicken Grenzen auf, die Anwendungen der DHFLC-Geräte einschränken können. Die Anmeldung betrifft FLC-Gemische aus 5-Alkyl-2-(p-alkoxyphenyl)pyrimidinen und einem chiralen nicht-racemischen Terphenyldiester (siehe auch die EP-Anmeldung 339 414).EP Application 309,774 relates to DHFLC display cells with chiral smectic FLCs whose helical structure can be altered by an electric field to alter their optical anisotropy. The application relates to DHF cells in which the ratio of the cell thickness to the pitch of the helix (d / p) of the FLC is more than 5 and preferably more than 10, which is between 22.5 ° C and 50 ° C, and where it is obviously enforced that a so-called phase factor

greater than 0.1 and preferably greater than 0.45. This requirement places limits on cell thicknesses that may limit applications of DHFLC devices. The application relates to FLC mixtures of 5-alkyl-2- (p-alkoxyphenyl) pyrimidines and a chiral non-racemic terphenyl diester (see also EP Application 339 414).

EP Application 405 346 relates to bistable FLC cells having aligned FLCs with an achiral smectic C-host material mixture and a dopant that induces a pitch less than 1 μm. It is reported that the mixtures have a spontaneous polarization greater than 10 nC / cm ² and a value Θ greater than 10 °. Upon successful formation in an electric field, the cell shows dark parallel lines at zero voltage between crossed polarizers, indicating a non-homogeneous Structure is.

EP application 404 081 relates to high polarization and short pitch FLC elements. This application relates to the use of short C ^* pitch materials wherein p is at least less than ½ d. in SSFLC cells, to eliminate the optical hysteresis observed in cells with high spontaneous polarization. Mixtures with a C ^* pitch in the range of 0.25 μm to 0.63 μm are reported, but the shortest pitch at room temperature was about 0.39 μm. It has been reported that the short C ^* pitch FLC blends have N ^* pitches that are at least greater than 8 μm.

Funfschilling and Schadt (1989) J. Appl. Phys. 66 (8): 3877-3882, report quickly appealing, for the multiplex operation suitable DHFLC displays. The authors share with that DHF cells FLCs with both a very short pitch, which is much smaller than the cell thickness, as well as weak surface interactions in the Cell require. It is communicated that several methods of manufacture reduce the tendency of the helix to unwind: the action a shear, the use of different directions of rubbing on the upper and the lower plate of the cell and surface treatments, which lead to zig zag defects in the SSFLCs. However, these treatments are practicing an adverse effect on the optical contrast of the cell. They report about DHF cells produced by rubbing the alignment layers in one Direction (unidirectional rubbing) were made with a Contrast Ratio (ON / OFF) from 12: 1 and over 40: 1 sheared cells of DHF cells Production of high-contrast electro-optical DHFLC devices problematic.

Summary the invention

It is an object of the present invention to provide FLC compositions having liquid crystal phase properties, short ferroelectric phase pitch, long N ^* pitch, and high spontaneous polarization useful in electro-optical FLC devices that have a short ferroelectric phase And which are of particular use in electro-optical FLC devices that require the presence of a helical director structure in the FLC.

The The problem underlying the invention was achieved by the ferroelectric Liquid crystal material according to claims 1 to 22 solved.

To achieve these objects, the present invention provides high-contrast electro-optical devices containing non-surface-stabilized FLC cells that retain a helical director structure (n ^) and a change in optical anisotropy as a function of the magnitude of the applied electric field or drive voltage demonstrate. In particular, the present invention provides high-contrast DHF-effect cells. The FLC cells comprise chiral ferroelectric liquid crystals having a ferroelectric phase, for example a smectic C ^* phase, and a chiral nematic (N ^* -) phase at temperatures above the ferroelectric phase. The FLC cells comprise evenly spaced plates containing electrodes between which the FLC is aligned. At least one of the electrode plates is transparent or semitransparent. One of the electrode plates may be reflective, forming a DHF cell operating in the reflection mode. The cell is provided with means for detecting the change in optical anisotropy induced by the application of the electric field in the FLC; For example, in a cell operating in the transmission mode, a polarizer at the input and a polarization analyzer may be placed on both sides of the electrode plates.

One Means for aligning the FLC may be in contact within the cell provided with the FLC, for example, on the inner surfaces the electrode plates. The means for aligning may be an alignment layer be that before inserting of the FLC between the plates the unidirectional, parallel or antiparallel, rubbing or brushing is subjected. For example, nylon, polyimide or polyamide layers deposited on the plates or spun on the plates. That's weird Depositing SiO cells is an alternative means of alignment. Alignment layers of grated nylon are used for manufacturing high contrast DHFLC cells.

The FLC layers of the devices are preferably aligned planar and most preferably in the "bookshelf" geometry. In the FLC devices, the pitch of the natural helix of the FLC in the ferroelectric phase is sufficiently shorter than the thickness of the ferroelectric liquid crystal layer, ie, the thickness (d) of the FLC cell, that the ferroelectric liquid crystal layer in the device Helical director structure and thus is not surface stabilized. In the N ^* phase of the FLCs, to facilitate alignment of the FLC in the device, the pitch of the natural helix is sufficiently greater than the cell thickness so that the device has a high optical contrast of at least about 40: 1, and more preferably 100: 1 or more. In order to further facilitate good alignment, it is desirable that the FLC also has an orthogonal smectic phase as an intermediate with respect to the temperature between the ferroelectric phase and the N ^* phase. The orthogonal smectic phases include, among others, the smectic A phase and the smectic B phase. The presence of the smectic A phase is preferred.

High contrast electro-optical devices preferably comprise FLC materials wherein the size of the pitch of the natural helix in the ferroelectric phase is less than about 1/5 d (the thickness of the FLC cell) and the size of the pitch of the natural helix in the N ^* - Phase is at least about d. However, it is more desirable that the size of the pitch of the natural helix in the ferroelectric phase be smaller than about 1/10 d. The pitch of the natural helix in the N ^* phase is preferably longer than about 4 d, and more preferably longer than 8 d.

It is preferred that the FLCs of the present invention have ferroelectric phases with the desired pitch properties at applicable device operating temperatures. Applicable operating temperatures for an FLC or DHFLC device depend on the desired application of the device. In some cases, operation at room temperature (20 ° C to 30 ° C) or below (10 ° C to 30 ° C) is desired. In other cases, such as projection devices, higher operating temperatures (50 ° C to 80 ° C) are desirable. Thus, the applicable operating temperatures for devices are in the range of about 10 ° C to 80 ° C. The FLCs of the present invention preferably exhibit the desired long N ^* pitch properties up to about 1-2 ° C above the transition temperature into the N * phase. More preferably, the FLCs of the present invention exhibit the desired long N ^* pitch at at least up to 5 ° C above the N ^* transition point. Most preferably, the FLCs of the present invention indicate the desired long N ^* pitch throughout the N ^* phase.

Ferroelectric phases of the present invention are chiral tilted smectic phases, including the smectic phases C ^* , I ^* , F ^* , H ^* , J ^* and G ^* , all of which exhibit ferroelectric properties {Meyer et al. (1975), J. de Physique 36, L-69}. The preferred ferroelectric phase of the FLCs of the present invention is a smectic C ^* phase.

orthogonal smectic phases of the present invention are those in those the long molecular axis the FLC compounds average perpendicular to the smectic layers run, including the smectic A and B-phases.

For use in the visible wavelength range, thicknesses of FLC cells between about 0.5 and 6.0 μm are preferred, depending on the birefringence of the FLC. To avoid diffraction effects and to avoid surface stabilization effects in DHFLC applications in the visible, it is preferred that the pitch of the natural helix of the FLC in the ferroelectric phase be substantially smaller than the wavelength of the light to be used and substantially smaller than the cell thickness is. A pitch in the ferroelectric phase with an amount in the range of less than 0.1 to 1.2 microns is desirable. A pitch in the ferroelectric phase of less than about 0.25 μm is more desirable, and an amount less than about 0.15 μm is most desirable. A natural N ^* pitch of greater than 4 d is desirable in such cells and more than 8 d is more desirable. The preferred ferroelectric phase is the smectic C ^* phase. Again, for DHFLC devices that are visibly useful, it is preferred that the FLCs of the device exhibit the desired ferroelectric phase and pitch at an applicable operating temperature and have an N ^* pitch up to about 5 ° C above the transition to the N ^* Show phase. Again, it is preferred that the FLCs have a smectic A phase between a smectic C and the chiral nematic phase.

To achieve the high-contrast devices, particularly high-contrast DHFLC devices, FLC compositions comprising at least two components and comprising a ferroelectric phase and an N ^* phase at temperatures above the ferroelectric phase, wherein the pitch of the natural helix in the ferroelectric phase is less than about 1/5 d, preferably less than about 1/10 d, and the pitch of the natural helix in the chiral nematic phase is equal to or greater than d, preferably greater than 4 d, and more preferably greater than 8 d. Preferred compositions are those having a ferroelectric smectic C ^* phase.

FLC compositions of the present invention comprise a chiral dopant that induces a short natural helix pitch in the ferroelectric phase but does not induce a short natural helix pitch in the chiral nematic phase of the FLC composition. FLC compositions of the present invention further comprise a host material which itself has a tilted smectic phase and may be chiral or achiral. The hosts most preferably have a nematic phase at temperatures above that of the tilted smectic phase, and preferably also have an orthogonal smectic intermediate phase intermediate in temperature between those of the tilted smectic phase and the nematic phase. In the FLC composition, the pitch of the natural helix in the N ^* phase is at least 4 times longer, and preferably 10 times longer, than that in the ferroelectric phase. The FLC compositions may contain more than one short pitch inducing chiral dopant, preferably all of which contain the same sign of the pitch of the ferroelectric phase. To maximize the N ^* pitch, it is desirable that not all of the combined chiral dopants in the FLC compositions have the same sign of N ^* pitch. The host materials can usually be mixtures of components and are usually those as well. The host materials are typically achiral, but may be chiral non-racemic FLCs with low spontaneous polarization. When a chiral non-racemic FLC host is employed, it is preferred that the FLC host and the short pitch-inducing FLC dopants combined together have the same sign of ferroelectric pitch.

Preferred FLC compositions of the present invention exhibit the following phase sequence with decreasing temperature: I, N ^* , orthogonal-smectic, ferroelectric, X (where I is isotropic, N ^{* is} chiral nematic, and X is crystalline). More preferably, the decreasing temperature FLC compositions show the phase sequence I, N ^* , A, C ^* , X (where I = isotropic, N ^* = chiral nematic, A = smectic A, C ^* = smectic C ^* and X = crystalline).

The FLC compositions of the present invention may also contain a nematic phase pitch compensation agent or mixtures of such agents, all preferably having the opposite sign of the N ^* pitch (ie, the helical twist direction) as that of the FLC mixture Host and the short pitch inducing FLC dopant.

preferred FLC compositions of the present invention contain less as about 30% by weight of the short pitch-inducing chiral dopant or less than about 5 to 10% by weight of the nematic compensating agents Pitch. More preferred compositions contain less than about 20 % By weight of a short pitch inducing dopant. More preferred compositions contain less than about 1% by weight of the nematic pitch compensation agents.

In preferred FLC compositions of the present invention, the size of the spontaneous polarization is greater than about 10 nC / cm ² . Preferred FLC compositions have tilt angles greater than about 10 °, more preferably greater than about 18 °, and most preferably equal to or greater than 22.5 °.

worin
^* ein asymmetrisches Kohlenstoff-Atom darstellt,
X und Y unabhängig voneinander Halogene sind und
R' eine Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen ist, in der ein oder mehrere, einander nicht-benachbarte Kohlenstoff-Atome durch eine Doppelbindung, ein O-Atom, ein S-Atom oder eine Si(CH₃)₂-Gruppe ersetzt sein können, oder
R' eine Acyl-Gruppe -OCO-R'' ist, in der
R'' eine Gruppe mit 2 bis etwa 20 Kohlenstoff-Atomen ist, in der ein oder mehrere, einander nichtbenachbarte Kohlenstoff-Atome durch eine Doppelbindung, ein O-Atom, ein S-Atom oder eine Si(CH₃)₂-Gruppe ersetzt sein können.
R' kann geradkettige oder verzweigte Alkyl- oder Alkenyl-Teile sowie Cycloalkyl- oder Cycloalkenyl-Teile enthalten. Bevorzugte chirale Dotierungsmittel sind solche, in denen X und Y unabhängig voneinander F oder Cl sind, und mehr bevorzugte FLC-Dotierungsmittel sind diejenigen, in denen X und Y beide F sind. Im allgemeinen können der Flüssigkristall-Kern und das chirale Dotierungsmittel beliebige mesomorphe Gruppen sein, die mit der Induzierung einer kurzen Ganghöhe kompatibel sind, beispielsweise solche, die aromatische oder Cyclohexyl-Gruppen enthalten. Zwei- und Drei-Ring-Kerne sind bevorzugt. Die zweite Schwanz-Gruppe kann im allgemeinen chiral oder achiral sein und enthält typischerweise 1 bis etwa 20 Kohlenstoff-Atome und kann, unter anderem, ein geradkettiges oder verzweigtes Alkyl, Alken, Cycloalkyl, Cycloalkenyl, Ether, Ester, Acyl, Thioether, Thioester oder eine Alkylsilyl-Gruppe sein.Chiral dopants are chiral nonracemic compounds having a liquid crystal core, a second tail group, and a chiral substituted 2,3-dihaloalkyloxy tail of the formula

wherein
^{* represents} an asymmetric carbon atom,
X and Y are independently halogens and
R 'is a group of 1 to about 20 carbon atoms in which one or more non-adjacent carbon atoms are represented by a double bond, O atom, S atom, or Si (CH ₃ ) ₂ group can be replaced, or
R 'is an acyl group -OCO-R "in which
R '' is a group having 2 to about 20 carbon atoms in which one or more non-adjacent carbon atoms is replaced by a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group could be.
R 'may contain straight or branched alkyl or alkenyl moieties, as well as cycloalkyl or cycloalkenyl moieties. Preferred chiral dopants are those in which X and Y independently of one another are F or Cl and more preferred FLC dopants are those in which X and Y are both F. In general, the liquid crystal core and the chiral dopant may be any mesomorphic groups compatible with short pitch induction, for example, those containing aromatic or cyclohexyl groups. Two- and three-ring cores are preferred. The second tail group may be generally chiral or achiral and typically contains from 1 to about 20 carbon atoms and may include, but are not limited to, straight chain or branched alkyl, alkene, cycloalkyl, cycloalkenyl, ethers, esters, acyl, thioethers, thioesters or the like an alkylsilyl group.

Short-pitch-inducing chiral dopants of the present invention are chiral non-racemic compounds of the formula RAZ ₁ -BOM, in the
A and B are independently selected from 1,4-phenylene, 1,4-phenylene in which one or two of the ring carbon atoms are replaced by nitrogen atoms, or 1,4-cyclohexylene, and Z ₁ is one Single bond, an O atom, a -CO-O- or an -O-CO- group.

R ist eine Gruppe mit 1 bis 20 Kohlenstoff-Atomen, in der ein oder mehrere, einander nicht-benachbarte Kohlenstoff-Atome durch eine Doppelbindung, ein O-Atom, ein S-Atom oder eine Si(CH₃)₂-Gruppe ersetzt sein können; R kann geradkettige oder verzweigte Alkyl- Alkenyl, Cycloalkyl- oder Cycloalkenyl-Teile enthalten, und
O-M ist eine 2,3-Dihalogenalkyloxy-Struktureinheit der Formel

worin
^* ein asymmetrisches Kohlenstoff-Atom darstellt,
X und Y unabhängig voneinander Halogene sind und
R' eine Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen ist, in der ein oder mehrere, einander nichtbenachbarte Kohlenstoff-Atome durch eine Doppelbindung, ein O-Atom, ein S-Atom oder eine Si(CH₃)₂-Gruppe ersetzt sein können, oder eine Acyl-Gruppe -OCO-R'', worin
R'' eine Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen ist, in der ein oder mehrere, einander nicht-benachbarte Kohlenstoff-Atome durch eine Doppelbindung, ein O-Atom, ein S-Atom oder eine Si(CH₃)₂ Gruppe ersetzt sein können.
R' und R'' können geradkettige oder verzweigte Alkyl-Alkenyl, Cycloalkyl- oder Cycloalkenyl-Teile enthalten.For example, -AZ ₁ -B- may include:

R is a group of 1 to 20 carbon atoms in which one or more non-adjacent carbon atoms are replaced by a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group can; R may contain straight-chain or branched alkyl-alkenyl, cycloalkyl or cycloalkenyl parts, and
OM is a 2,3-dihaloalkyloxy structural unit of the formula

wherein
^{* represents} an asymmetric carbon atom,
X and Y are independently halogens and
R 'is a group of 1 to about 20 carbon atoms in which one or more non-adjacent carbon atoms are replaced by a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group may, or an acyl group -OCO-R ", in which
R '' is a group of 1 to about 20 carbon atoms in which one or more non-adjacent carbon atoms are represented by a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group can be replaced.
R 'and R "may contain straight-chain or branched alkyl-alkenyl, cycloalkyl or cycloalkenyl moieties.

in der
^* ein asymmetrisches Kohlenstoff-Atom darstellt,
Z₁ eine Einfachbindung ist,
B ein 1,4-Phenylen ist und
A ein 2,5-Phenylpyrimidin ist, z.B. kann -A-Z₁-B-

und worin
R' eine Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen oder eine Acyl-Gruppe -OCO-R'' mit 1 bis etwa 20 Kohlenstoff-Atomen ist.In other specific embodiments, short-pitch-inducing chiral dopants of the present invention are chiral non-racemic compounds of the formula

in the
^{* represents} an asymmetric carbon atom,
Z _{1 is} a single bond,
B is a 1,4-phenylene and
A is a 2,5-phenylpyrimidine, for example, -AZ ₁ -B-

and in which
R 'is a group having 1 to about 20 carbon atoms or an acyl group -OCO-R "having 1 to about 20 carbon atoms.

In special embodiments In the present invention, R 'may be an alkyl group of from 3 to about 20 Carbon atoms, an ω-monoalkene having from 3 to about 20 carbon atoms, an alkylsilyl group having 4 to about 22 carbon atoms or an acyl group -OCO-R "in which R" is a Alkyl group. with 3 to about 20 carbon atoms, a ω-monoalkene with 3 to about 20 carbon atoms or an alkylsilyl group with 4 to about 22 carbon atoms.

In other special embodiments In the present invention, R 'may be an alkyl group of 1 to about 20 Be carbon atoms or an acyl group -OCO-R ", in the R '' is an alkyl group with 1 to about 20 carbon atoms. Preferred alkyl groups R 'and R "have about 6 to about 12 carbon atoms.

R' ist eine Alkyl-Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen, eine ω-Alken-Gruppe mit 2 bis etwa 20 Kohlenstoff-Atomen, eine Alkylsilyl-Gruppe mit 4 bis etwa 22 Kohlenstoff-Rtomen oder eine Acyl-Gruppe -OCO-R'', in der R'' eine Alkyl-Gruppe mit 1 bis etwa 20 Kohlenstoff-Atomen, eine ω-Alken-Gruppe mit 2 bis etwa 20 Kohlenstoff-Atomen oder eine Alkylsilyl-Gruppe mit 4 bis etwa 22 Kohlenstoff-Atomen sein kann. Bevorzugte Alkyl-Gruppen für R' und R'' sind solche, die etwa 6 bis 12 Kohlenstoff-Atome haben.In a more specific embodiment, the core is -AZ ₁ -B-

R 'is an alkyl group of 1 to about 20 carbon atoms, an omega-alkene group of 2 to about 20 carbon atoms, an alkylsilyl group of 4 to about 22 carbon atoms, or an acyl group -OCO R '', in the R '' is an alkyl group having 1 to about 20 carbon atoms, an ω-alkene group having 2 to about 20 carbon atoms or an alkylsilyl group having 4 to about 22 carbon atoms can be. Preferred alkyl groups for R 'and R "are those having about 6 to 12 carbon atoms.

More are preferred to FLC compositions with a higher spontaneous To obtain polarization, chiral dopants with 2,3-difluoroalkyl tails, in the configuration of the asymmetric carbon atoms 2 (R), 3 (R) or 2 (S), 3 (S).

Hosts employable in the devices of the present invention comprise at least about 10% by weight of one or more dioxy-substituted compounds of the formula R ₁ -OCD-OR ₂ , in the
one of the groups C or D is a 1,4-phenylene and the other of the groups C or D is a 1,4-phenylene in which one or more of the carbon atoms of the ring are each replaced by a nitrogen atom, and wherein R ₁ and R _{2 are} independently alkyl or alkenyl groups having 1 to about 20 carbon atoms or an alkylsilyl group having 4 to about 22 carbon atoms. R ₁ and R ₂ may contain straight-chain or branched alkyl, alkenyl, cycloalkyl or cycloalkenyl moieties. Preferred host components of the present invention are phenylpyrimidines in which one of the groups C or D is a 1,4-phenylene and the other of the groups C or D is a 2,5-pyrimidine. In specific embodiments of the present invention, R ₁ and R _{2 are} alkyl groups or ω-monoalkenes of 1 to about 20 carbon atoms or silylalkyl groups having a terminal -Si (CH ₃ ) ₃ group of 4 to about 22 carbon atoms. atoms. Preferred alkyl groups R ₁ and R ₂ have from about 6 to about 12 carbon atoms.

In a specific embodiment, hosts of the present invention comprise at least about 10% by weight of a compound of the formula R ₁ -OCD-OR ₂ , in the
C is 2,5-pyrimidine and D is 1,4-phenylene and wherein R ₁ and R _{2 are} independently alkyl or omega-alkenyl groups having from 3 to about 20 carbon atoms.

Hosts of the present invention may also use one or more of the compounds of the following formulas R ₃ -EFZ ₂ -R ₄ or R ₅ -GHZ ₃ -Cyc-R ₆ in which
R ₃ , R ₄ , R ₅ and R _{6 are} independently alkyl or alkenyl groups having from about 1 to about 20 carbon atoms or an alkylsilyl group having from about 4 to about 22 carbon atoms. The groups R ₃ , R ₄ , R ₅ and R ₆ may contain straight-chain or branched alkyl, alkenyl, cycloalkyl or cycloalkenyl moieties.

One of the groups E or F is 1,4-phenylene and the other of the groups E or F is 1,4-phenylene wherein one or two of the ring carbon atoms are replaced by a nitrogen atom and Z is ₂ either an oxygen atom, a -CO-O- or an -O-CO- group. One of the groups G or H is 1,4-phenylene and the other of the groups G or H is 1,4-phenylene wherein one or two of the ring carbon atoms are replaced by a nitrogen atom and Z is ₃ a single bond, an oxygen atom, an -O-CO- or a -CO-O- group. Cyc is a 1,4-cyclohexyl group.

-EF- and -GH- are preferably phenylpyrimidines, including

Z ₂ is preferably an oxygen atom. Z ₃ is preferably a single bond or an -O-CO- group, most preferably an -O-CO- group. Cyc is preferably a trans-1,4-cyclohexenyl group. R ₃ , R ₄ , R ₅ and R ₆ are preferably alkyl or omega-alkenyl groups having from 3 to about 20 carbon atoms, and more preferably alkyl groups having from about 6 to 12 carbon atoms.

Preferred host mixtures of the present invention comprise about 50% by weight or more of compounds of the formula R ₁ -OCDOR ₂ or about 50% by weight of a mixture of compounds of the formulas R ₁ -OCDOR ₂ or R ₃ -EFOR ₄ wherein at least 10% by weight of a compound of the formula R ₁ -OCDOR ₂ .

Preferred host mixtures also comprise at least about 10% by weight of compounds of the formula R ₅ -GHO-CO-Cyc-R ₆ .

The present invention also provides methods for analyzing narrower chiral non-racemic compounds for their ability to induce a short pitch of the ferroelectric phase in mixtures that retain a long N ^* pitch. Analogous methods for identifying hosts suitable for use in mixtures of the present invention with short ferroelectric pitch and long N ^* pitch are also provided.

The present invention further provides methods for producing high contrast DHF cells, particularly high contrast cells that are half wave plates. Such DHF cells are prepared by subjecting a FLC composition of the present invention having a short pitch of the ferroelectric phase at operating temperatures of the device and a long N ^* at temperatures of at least 1-2 ° C above the N ^* transition point. Has pitch, is inserted between evenly spaced electrode plates. The electrode plates are surface treated to provide an alignment layer, such as a unidirectionally rubbed polymer layer. The FLC is heated to at least 5 ° above the transition point into the N ^* phase, inserted between the surface treated electrode plates, and slowly cooled to the ferroelectric phase at a rate of 0.05-2 ° C / min to achieve good alignment of the FLC to achieve.

FLC compositions of the present invention in any FLC device be used, which involves the use of a material of short pitch of the ferroelectric Phase requires or benefits from it. The FLC compositions of the present invention in devices with the so-called anti-ferroelectric effect and in bistable SSFLC-like cells are used. The present invention also makes available FLC devices with FLC layers that FLC compositions of the present invention.

Short description the figures

1 Figure 11 shows a schematic representation of a DHFLC cell with smectic C ^* layers perpendicular to the electrode plates in the bookshelf geometry. The helical axis runs along n ^, the normal of the smectic layer parallel to the electrode plates. The pitch of the helix (p) is shown. For DHF effect cells, the cell thickness (d) is much larger than the size of the C ^* pitch.

2 Figure 9 is a graphical representation of the linear optical response of a planar-aligned DHFLC cell of the present invention upon application of a linear drive voltage. The DHFL cell is 2.5 μm thick and contains the DHFLC mixture 1 from Example 4.

3 Figure 2 shows a comparison of the alignment of a DHFLC cell (2.5 μm cell with mixture 1), View A, with a 2.5 μm DHFLC cell using a commercially available DHF mixture 5679 (Hoffman La Roche), View B. Views A and B are photomicrographs of the texture of the cells. The DHFLC cell containing the mixture 1 of the present invention is much smoother in texture than the prior art DHFLC mixture, which is an indication of the better alignment of the view A cell. The DHFLC cell containing mixture 1 has a contrast ratio of 158: 1 while the 5679 containing cell has a contrast ratio of 25: 1.

4 Figure 4 is a graphical representation of the magnitude of the pitch over temperature for a representative FLC blend of the present invention, Blend 1 (MX5565). The change in C ^* pitch (empty squares) and N ^* pitch (solid squares) in μm as a function of temperature (° C) is shown. The scale for the N ^* pitch (y-axis, right) is 10 times greater than the scale for the C ^* pitch (y-axis, left). The temperature ranges of the smectic A- (A), the smectic C ^* (C ^* ), the chiral nematic (N ^* ) and the isotropic (I) phase are shown with the transition points indicated by dashed lines in the graph. The actual values of C ^* pitch and N ^* pitch in Mixture 1 are both negative. In the 4 The absolute amounts of pitch were used to compare the change in the size of the pitch as a function of temperature.

Full Description of the invention

It is well known in the art that improved alignment and contrast ratio in SSFLC cells can be alleviated by an FLC having a long-cycle N ^* phase For example, in order to facilitate alignment in SSFLCs, the N ^* pitch should be at least d, and preferably 4d or greater, in the prior art It is also well known that for the production of SSFLC cells, cell alignment is further facilitated by the presence of a smectic A phase intermediate in the temperature range between the chiral tilted smectic ferroelectric phase and the N ^* phase in the FLC However, it is believed that SSFLC cells require a relatively long pitch (typically longer than d, and preferably longer than 4d) in their ferroelectric phase.

It is difficult to achieve high contrast in DHFLC devices with prior art methods and compositions. The contrast ratio is defined as the ratio of transmitted light in the ON state (maximum amount of white light passing through the device) to that in the OFF state (minimum amount of white light passing through the device). For maximum contrast, a DHFLC cell half-wave plate is positioned between crossed (liner) polarizers and oriented so that minimal permeability is observed upon application of E _max (or -E _max ). A maximum contrast is obtained when the voltage level applied to the cell rotates the optical axis by a total of 45 ° between the ON state and the OFF state. Most often, the contrast is limited by light filtering in the OFF state. In addition, the maximum transmission in the ON state is limited by a maximum rotation of the optical axis by less than 45 °, since FLCs often have angles of inclination smaller than 22.5 °.

A minimum transmittance in the OFF state requires a good uniform alignment of the DHFLC inside the cells between the electrode plates. Very little is reported in the art on improving alignment in DHFLC cells. Funfschilling and Schadt (1989) (loc. Cit.) Suggest that the contrast of DHFLC cells can be improved, but give no instructions on how to achieve a contrast greater than 40: 1. They used shear alignment techniques to obtain DHFLC cells with a contrast ratio of 40: 1. However, shear alignment techniques are not very practical for large-scale manufacturing. More practical alignment techniques using unidirectional rubbing or brushing of the alignment layers have been reported to give contrast ratios in DHFLC cells as low as 12: 1. The EP 309 774 refers to a DHF cell with a contrast ratio of 100: 1, but gives no details of how this contrast was achieved.

The inventors have found that procedures analogous to those that can be used in improving alignment and contrast in SSFLC cells to improve alignment and contrast in DHFLC cells. The inventors have found that an N ^* pitch that is sufficiently long to achieve alignment and, more particularly, planar alignment and "bookshelf" placement of a DHFLC cell in FLC compositions can also be achieved with a short pitch possess ferroelectric phase, which is necessary for DHFLC cells that are stable against unwinding of the helix. FLC compositions having a short pitch ferroelectric phase, eg, a smectic C ^* phase, and a long pitch N ^* phase at higher temperatures may be aligned using procedures such as those described in WO 87/06021. These procedures combine treatment of the cell surface (ie, the alignment layers) with the cooling of the FLC in contact with the treated surfaces of the cell plates from the nematic phase to the ferroelectric phase. Good FLC alignment and high DHFLC cell contrast result. High contrast DHFLC cells, ie 40: 1 or higher, and preferably 100: 1 or higher, can be prepared by these procedures. The cooling of the FLC from the nematic phase to the ferroelectric phase may be accompanied by the application of an electric field across the FLC layer. It has also been found that the presence of an orthogonal smectic phase, such as a smectic A phase, an intermediate in temperature between the nematic and ferroelectric phases, further promotes good alignment and the production of high contrast cells.

In particular, the inventors have discovered chiral nonracemic dopants which, in combination with suitable hosts, induce a short pitch of the ferroelectric phase without inducing a short pitch in chiral nematic phases. Both the FLC host and the chiral non-racemic short pitch dopant were found to affect the relative sizes of the ferroelectric, ie, smectic C ^* and N ^* phase pitches.

As usual, a ferroelectric liquid crystal cell comprises an FLC layer of known thickness (d) between holding plates. Such as in the DHFLC cell 10 of the 1 is shown has a cell Plat th 11 and 12 and an FLC layer 15 , The plates may be made of glass, plastic or other material having suitable optical properties and are substantially parallel to each other. At least one of the plates is transparent or semi-transparent. In a device operating in the transmission mode, both plates are typically transparent or semi-transparent. In a device operating in the reflection mode, one of the plates has a reflective surface, such as a metal foil or a deposited reflective surface. The plates are provided with electrodes so that a voltage can be applied to the electrodes to create an electric field across the FLC layer. For example, transparent or semi-transparent conductive layers ( 13 . 14 ) be applied to the inner surface of the plates, as in turn 1 is shown. Suitable transparent or semi-transparent electrode materials include, among others, tin oxide and indium tin oxide. In a device operating in reflection mode using a metal mirror, the reflective surface may also serve as an electrode. The electrode may be a single adjacent layer provided, for example, as a layer deposited on a plate as a whole, or it may be deposited in a desired pattern, for example, mounted in the form of a grid over the plate to form pixels. Spacers can be used between the plates to obtain the desired FLC thickness.

FLC cells may be provided with alignment layers on the internal surfaces of the electrode plates in contact with the FLC layer to induce a desired FLC alignment or to assist or enhance FLC alignment within the cell. To achieve high alignment and high contrast, the use of alignment layers is preferred. For example, a nylon, polyimide, polyamide, or other polymeric layer may be applied to the interior surfaces of the panels. Such alignment layers are rubbed or brushed in one direction (unidirectional rubbing), for example with a soft rayon cloth. The layers on opposite plates are rubbed in the same direction, but the rubbing on the opposite plates may be parallel or anti-parallel. The provision of a rubbed or brushed alignment layer on the cell plates is also referred to as treatment of the cell surfaces. These surface treatments cause, assist or enhance the alignment of the FLC molecules in the FLC layer, as recognized in the art. Other techniques of alignment for FLCs include shearing operations and oblique deposition of SiO _x on the inner surfaces of the cell plates. The techniques of alignment can be used to achieve a desired alignment, for example, either planar or homeotropic alignment appropriate for a particular application.

DHFLC cells are FLC cells that retain the helical director structure within the FLC layer and are thus not surface stabilized. The helix director structure of a DHF cell is in 1 illustrated. Within a smectic layer, the middle long molecular axis n ^ is oriented at an angle (Ψ) to the normal of the layer ẑ. Ferroelectric materials have a spontaneous polarization P that is parallel to the layer planes and perpendicular to n ^. In aligned FLCs, n ^ and P follow a worm movement around ẑ from smectic layer to smectic layer, as in FIG 1 is specified. P and n ^ of successive layers make a worm movement over an azimuthal angle φ, wherein they correspond to those in the 1 form helix director structure represented. The layer normal is parallel to the helical axis. The distance along the helix axis between places where n ^ and P have the same orientations, ie for a change of φ by 360 °, is the amount of pitch. The pitch may be + or -, indicating opposite twist directions of the helix. The helical structure of the DHFLC layer may be present substantially throughout the FLC layer or only in a central portion of the FLC layer. A sufficient helical structure must be present in the layer for a given cell to exhibit a DHF effect. For example, the helix director structure of the FLC layer may be torn in the vicinity of the board surfaces due to surface interactions. Preferred DHFLC cells are those in which the presence of the helix structure of the FLC layer is maximized. The contrast of the DHFLC cell is enhanced by good molecular alignment of the FLC. The techniques for aligning for DHF cells should be such as to facilitate alignment without completely disrupting the helical structure. Preferred techniques for aligning for DHF cells are those that enable good alignment without significant disruption of the helical director structure. The inventors have found that the use of parallel rubbed nylon alignment layers is preferred for the production of high contrast DHF effect cells. As discussed above, the DHF effect is now well known in the art. A DHFLC cell or a DHFLC device containing such a cell is one in which the application of an incremental voltage change to the cell electrodes causes an incremental distortion of the helix structure of the FLC layer, and as a result of this distortion there is an incremental one Change in the detected optical anisotropy of the cell. According to the state of the art, a DHFLC cell exhibits an incremental change in the orientation of its optical axis as well as increments Change the birefringence with an incremental change in stress. Both of these effects contribute to the proven change in optical anisotropy as a function of stress.

FLCs, which are half-wave plates, have particular advantages for use in electro-optical devices. Half wave plates are designed so that

Half-wave plate FLC cells require lower operating voltages and are suitable for broadband applications more useful as cells higher Order (e.g., 3 / 2λ, etc.).

The change the permeability a half wave FLC cell with the wavelength is considerable lower than higher cells Order. This is especially true for applications in the visible to where an FLC half-wave plate designed so that Δnd = λ / 2 at about λ = 550 nm applies, a substantially achromatic half wave plate over the is the entire visible spectrum.

The Mixtures of the present invention with very short ferroelectric pitch allow the Construction of DHF cells, which are stable against unwinding and designed as a half-wave can be (Δnd = λ / 2).

The N ^* and ferroelectric pitch may be measured by any method known in the art. The Cano-wedge method can be used to measure either the ferroelectric (ie smectic C ^* ) or N ^* pitch in samples having a longer pitch than about 0.5 to 0.6 μm. See: R. Cano (1967), Bull. Soc. France Mineral Crystallogr. XC 333; P. Kassubek and G. Meier (1969) Mol. Cryst. Liq. Cryst. 8: 305-315; Ph. Marinot-Lagarde et al. (1981) Mol. Cryst. Liq. Cryst. 75: 249-286. Selective measurements of the reflection of thick (about 400 μm) homeotropically aligned cells are typically used to measure pitches of less than about 0.5 μm in size. See: K. Kondo et al. (1982), Jpn. J. Appl. Phys. 21: 224.

The ferroelectric and N ^* pitch of an FLC material varies as a function of temperature. Typically, the ferroelectric pitch tends to decrease with decreasing temperature, and the N ^* pitch tends to infinity at the transition point between the N ^* phase and the lower temperature smectic phase. Often, the N ^* pitch drops very rapidly within a few tenths of degrees above the N ^* transition point. Nematic phase pitch compensation means can be used to extend the N ^* pitch at temperatures above the transition point. The compositions of the present invention combine a long N ^* pitch at at least 1-2 ° C above the transition point with a short ferroelectric pitch at applicable equipment operating temperatures (about 10 ° C to 80 ° C).

4 Figure 12 shows how the magnitudes of the C ^* and N ^* pitch of a representative mixture of the present invention change as a function of temperature. The N ^* pitch is in the 4 in a ten times larger scale than the C ^* pitch. The C ^* pitch and the N ^* pitch of Mix 1 are always negative {ie the pitch or twist is indicated by a minus sign). This mixture has an orthogonal smectic phase, here a smectic A phase, an intermediate between the chiral smectic phase (smectic C ^* ) and the chiral nematic phase. Going up from the transition A to N ^* , the N ^* pitch increases towards infinity as the temperature decreases. In this mixture, the N ^* pitch is about 5 μm at a temperature about 10 ° C above the AN ^* transition and about 7.5 μm at about 5 ° C above this transition and increases above about 2 ° C above this transition over about 35 microns too. As the temperature approaches the AN ^* transition point, the N ^* pitch increases toward infinity.

FLC blends of the present invention exhibit a long N ^* pitch at 1-2 ° C above the transition point into the N ^* phase, and preferably exhibit a long N ^* pitch up to at least 5 ° C above this transition point, and on Most preferably, they have a long N ^* pitch throughout the N ^* phase. The measurement of the N ^* pitch can be readily made at temperatures of about 1-2 ° C above the N ^* transition point or above. As the temperature approaches the transition point to a distance within about 1 ° C, it becomes difficult to obtain accurate measurements of the N ^* pitch.

Like also in 4 is shown, the C ^* pitch may change as a function of temperature. Mixture 1 has a C ^* pitch of less than 0.5 μm at room temperature (20 ° C to 30 ° C). FLC blends of the present invention preferably have a short pitch at temperatures useful for the operation of optical devices. The applicable operating temperatures vary depending on the type of device and its application. For some applications, short C ^* pitches at room temperature are desirable. In other applications, it may be necessary or desirable to operate at higher temperatures. In such cases, short C ^* pitch FLCs are desirable at temperatures above room temperature, eg, 40 ° C to 80 ° C.

High contrast DHF cells are prepared by introducing an FLC mixture of the present invention between spaced electrode plates. The inner surface of the electrode plates is preferably provided with a surface treatment that facilitates alignment, such as a unidirectionally rubbed polymer layer. The treated plates are oriented relative to each other so that the rubbing directions of the alignment layers on opposite plates are parallel or anti-parallel. The FLC mixture is introduced between the cell plates at a temperature which is at least about 5 ° C above the N ^* transition point. Preferably, the FLC mixture is introduced between the cell plates at a temperature at which it is in the isotropic phase. Then, the FLC is cooled to the N ^* phase and slowly cooled from a temperature at least about 5 ° C above the N ^* transition point to the ferroelectric phase. The FLC is cooled at a rate of 0.05-2 ° C / min from a temperature at least about 5 ° C above the N ^* transition point to the underlying smectic phase. When an orthogonal smectic phase is present, the FLC is slowly cooled through the orthogonal smectic phase to the lower temperature ferroelectric phase.

The resulting aligned DHFLC cell is then tested to assess the alignment, as is known in the art, for example, by measuring the contrast ratio of the cell. If desired, one may perform one or more additional steps of heating up to the N ^* phase and then slow cooling to the ferroelectric phase to achieve improved alignment. It is preferred that the operation of alignment be repeated until substantial alignment is achieved. A major orientation of a DHF cell is such that less than about 0.025% of the incident light seeps through the cell when placed between crossed polarizers in the minimum transmittance (OFF) position. A preferred orientation of a DHF cell is one which allows leakage of less than about 0.01 of the incident light in the OFF state. This mode of cell alignment results in a high contrast DGHF cell.

worin A, B, X, Y und R die im Vorstehenden angegebenen Bedeutungen haben und R' Alkyl; Alkenyl, Alkylsilyl oder eine andere Gruppe ist; wie. sie oben definiert ist, können nach Methoden, die in US-Patent 5 380 460 beschrieben sind, oder routinemäßigen Abänderungen dieser Methoden synthetisiert, werden. Verbindungen der Formel I, in denen R' eine Acyl-Gruppe ist, können nach Methoden, die in US 5 130 048 beschrieben sind, oder routinemäßigen Abänderungen dieser Methoden synthetisiert werden. Die oben zitierten Methoden können ohne weiteres mittels bekannter Hilfsmaßnahmen adaptiert werden, wie sie in den PCT-Anmeldungen WO 89/10356, WO 87/05015 oder WO 86/06401 oder dem US-Patent 4 886 622 für die Synthese von Verbindungen der Formel I beschrieben sind, die trans-1,4-Cyclohexylen-Gruppen enthalten.FLC compositions possessing phase and pitch properties useful in the devices are exemplified by compositions comprising a short pitch ferroelectric phase dopant with chiral non-racemic 2,3-dihaloalkoxytail. Groups such as those described in U.S. Patent Nos. 5,051,506, 5,380,460, and 5,130,048. Compounds of the formula I

wherein A, B, X, Y and R have the meanings given above and R 'is alkyl; Alkenyl, alkylsilyl or another group; as. as defined above can be synthesized by methods described in US Pat. No. 5,380,460 or by routine modifications of these methods. Compounds of formula I in which R 'is an acyl group can be prepared by methods described in US 5,130,048 described or routine modifications of these methods are synthesized. The above-cited methods can be readily adapted by known means as described in PCT applications WO 89/10356, WO 87/05015 or WO 86/06401 or US Pat. No. 4,886,622 for the synthesis of compounds of formula I are described which contain trans-1,4-cyclohexylene groups.

FLC compositions possessing phase and pitch properties used in the. Are exemplified by compositions containing short-period ferroelectric phase dopants and one or more hosts containing at least about 10% by weight of the dialkoxy compounds of formula II R ₁ OCD-OR ₂ II, in which R ₁ , C, D and R _{2 have} the meanings given above.

Compounds of formula II suitable as host components are either commercially available or may be prepared by well-known methods or by routine adaptation of well-known methods such as those described in EPO application EP 307 880 and the Swiss registration CH 593 495 are made available to be synthesized.

Hosts of the present invention may, in addition to the compounds of formula II, one or more of the compounds of formulas III and IV R ₃ -EFZ ₂ -R ₄ III or R ₅ -GHZ ₃ -Cyc-R ₆ IV in which R ₃ , R ₄ , R ₅ , R ₆ , E, F, Z ₂ , G, H, Z ₃ and Cyc have the meanings given above.

links of the formula III or IV used in the hosts of the present invention are either commercially available or can after Procedures well known in the art or by routine adaptation well-known method.

A chiral non-racemic dopant employable in the devices with short C ^* pitch inducing properties can be selected by routine testing according to the following procedure:
Mixtures of 10 to 50% by weight of the chiral non-racemic molecule (ie, the shortlisted dopant) to be tested with a room temperature smectic C or C ^* host compound containing at least 10% by weight of a dialkoxy compound of Formula II, preferably the smectic C hosts H ₁ and H ₂ (whose compositions are provided in the Examples) are prepared. Mixtures of acceptable dopants exhibit a smectic C ^* phase and a chiral nematic phase at higher temperatures with a certain dopant concentration between about 10 and 50 wt%. Shortlisted chiral non-racemic molecules which destroy the smectic C phase or the nematic phase in all such mixtures are not acceptable short-pitch dopants according to the present invention. The C ^* and N ^* pitch of the mixtures with the dopants to be tested are measured. The N ^* pitch is measured at temperatures above about 1 ° C to 2 ° C above the transition point to the N ^* phase. The C ^* pitch is measured within the range of the applicable operating temperatures of the equipment (about 10 ° C to about 80 ° C). Most typically, the N ^* pitch is measured at about 2 ° C above the N ^* transition point, and the C ^* pitch is measured at room temperature (20-30 ° C). Acceptable short pitch dopants are those in which in at least one of the blends the ratio N ^* pitch / C ^* pitch is about 1 or greater. Preferred short-pitch dopants are those in which, in at least one blend, the ratio N ^* pitch / C ^* pitch is about 4 or greater. For more preferred dopants, at least one of the test mixtures exhibits an N ^* / C ^* pitch ratio of about 10 or more. Preferred dopants are those in which at least one of the mixtures has a smectic C ^* pitch less than about 0.25 μm at room temperature and an N ^* pitch up to about 2 ° C above the transition point of about 2.5 μm or more shows. More preferred dopants are those in which at least one of the mixtures has a C ^* pitch of 0.15 μm or less at room temperature. Preferred dopants are those which induce the shortest C ^* pitch at dopant concentrations below about 10 to 20% (w / w) without lowering the N ^* pitch below about 4 μm. This procedure can be easily applied to select chiral dopants that induce a short ferroelectric pitch in any chiral tilted smectic phase.

A smectic C or C ^* host suitable for use in combination with dopants of the present invention inducing a short pitch can be selected by routine testing according to the following procedure: The shortlisted smectic C - or C ^* hosts show a smectic C or C ^* phase and preferably at higher temperatures a nematic phase. Hosts preferred for device applications have a smectic C or C ^* phase at applicable device operating temperatures (10 ° C to 50 ° C). Preferred shortlisted hosts have an orthogonal smectic phase, for example a smectic A phase, as an intermediate at temperatures between the smectic C or C ^* phase and the nematic phase. Mixtures of one too 10 to 50% of one of the specific exemplified short-cycle dopants according to the present invention, MDW128, MDW232, MDW116, MDW198, MDW316 or MDW317, as defined in the examples, are prepared. Hosts suitable for testing in accordance with this procedure must mix with at least one of the exemplified dopants at a dopant concentration of between about 10 and 50% (w / w). The C ^* pitch and the N ^* pitch of the mixtures with the hosts to be tested are measured. The N ^* pitch is measured at temperatures above about 1 ° C to 2 ° C above the N ^* transition point. The C ^* pitch is measured at applicable operating temperatures of the unit, approximately 10 ° C to 80 ° C. Most typically, the N ^* pitch is measured at 2 ° C above the transition point, and the C ^* pitch is measured at room temperature. Acceptable hosts for use in the compositions of the present invention are those in which the ratio N ^* pitch / C ^* pitch of at least one of the tested mixtures is about 1 or greater. Preferred hosts are those in which the ratio N ^* pitch / C ^* pitch of at least one of the mixtures is about 4 or greater. In more preferred hosts, the N ^* / C ^* pitch ratio is about 10 or more. Preferred hosts are those in which at least one of the mixtures has a smectic C ^* pitch less than about 0.25 μm at room temperature and an N ^* pitch up to about 2 ° C above the N ^* phase transition point of about 2 , 5 μm or more. More preferred hosts are those in which at least one of the mixtures has a C ^* pitch of 0.15 μm or less at room temperature.

This procedure can be easily applied to select any ferroelectric hosts. Once a short pitch-inducing chiral dopant has been identified by the procedure described hereinabove, this newly identified dopant can be used in the analytical testing for ferroelectric phase hosts in place of the dopant compounds specifically identified above. Due to mixing incompatibilities or other factors, a particular dopant of the present invention may not induce a short pitch in all ferroelectric, smectic C or smectic C ^* hosts.

hosts are typically achiral, or they are chiral non-racemic Materials with a low density of polarization.

The The following examples are for explanation the practice of the present invention, but are intended to their Limit scope in any way.

EXAMPLES

in denen R und R' die aufgeführten Bedeutungen haben,

in FCL-Wirts-Gemische eingeführt, und die C^*-Ganghöhe und die N^*-Ganghöhe der resultierenden Gemische wurden bestimmt. Die Messungen der Ganghöhe wurden mit Hilfe von Standard-Verfahren durchgeführt, mit der Cano-Keil-Technik oder mittels selektiver Reflexion, wie sie beispielsweise bei R. Cano (1967), loc.cit., oder von K.In the following examples, chiral FLC dopants of the formula

in which R and R 'have the meanings listed,

were introduced into FCL host mixtures, and the C ^* pitch and N ^* pitch of the resulting mixtures were determined. The measurements of the pitch were carried out by standard methods, by the Cano-wedge technique or by selective reflection, as described, for example, in R. Cano (1967), loc. Cit., Or by K.

Kondo et al. (1982), loc. Cit., Is described. For all phase diagrams the temperatures are given in ° C and I = isotropic, N ^* = chiral nematic, A = smectic A, C ^* = chiral smectic C, C = smectic C and X = crystalline.

example 1

Dependence of the C ^* and the N ^* -type level of the host-type

Blends of MDW116 with three types of FLC hosts were tested for C ^* and N ^* pitch as described above. The results of the study of the mixtures containing 10% by weight in these hosts are shown in Table 1.

Table 1

The host plays a dramatic role in inducing pitch in the mixture by the dopant. By changing the host, the C ^* pitch can be changed as much as 3-fold, and the N ^* pitch can be changed up to more than 5-fold.

Dialkoxy hosts provided the shortest C ^* pitch while maintaining a long N ^* pitch.

Example 2

FLC blends with short C ^* pitch

Difluoroalkoxy dopants induce a short pitch in dialkoxyphenylpyrimidine hosts. Table 2 summarizes measurements of C ^* and N ^* pitch of mixtures of 30% by weight of selected dopants in a selected dialkoxyphenylpyrimidine host H1.

Table 2

H1 is an achiral smectic C host material having the composition (in wt%):

H1 has the following phase diagram: I - 89 - N - 70 - A - 66 - C.

Example 3

Change in C ^* and N ^* pitch as a function of dopant concentration

Mixtures of MDW232 in a smectic C ^* host, H2, comprising dialkoxyphenylpyrimidines, were prepared and the C ^* and N ^* pitch of the mixtures were measured, respectively. As shown in Table 3, as the weight% in the mixture increases, the C ^* pitch becomes. shorter. The ratio N ^* pitch / C ^* pitch in these blends is at least 8, and in the blend at 30 weight%, the ratio is greater than 10.

Table 3

und sein Phasendiagramm ist I--93--N--78--A--71--C.H2 is an achiral smectic C host with the composition (in% by weight):

and its phase diagram is I-93-N-78-A-71-C.

Example 4

Mixtures with compensated N ^* pitch.

All of the following blends have a C ^* pitch less than 1 μm and an N ^* pitch greater than 10 μm.

Mixture 1 has the composition: 10% MDW198, 15% MDW116, 0.8% ZLI4572, 74.2% FELIX008.

Gemisch 1: C^*-Ganghöhe (25 °C) = -0,27μm; N^*-Ganghöhe (2 °C oberhalb des N^*-Phasenübergangs) = -38 μm; und das Phasendiagramm ist I--80--N^*--65--A--62--C^*--<25--X . ZLI4572 is a commercially available pitch compensation device with the phase diagram I-133-X:

Mixture 1: C ^* pitch (25 ° C) = -0.27μm; N ^* pitch (2 ° C above the N ^* phase transition) = -38 μm; and the phase diagram is I - 80 - N ^* - 65 - A - 62 - C ^* - <25 - X.

2 illustrates the DHF effect in a 2.5 micron thick DHF cell containing a bookshelf oriented layer of Mixture 1. The optical response in the cell is nearly linear with the applied electric drive field.

Mixture 2 has the composition: 10% MDW316, 10% MDW317, 0.7% ZLI4572, 79.3% FELIX008. Mixture 2: C ^* pitch (22 ° C) = -0.21μm; N ^* pitch = -30 μm; and the phase diagram is I - 72 - N ^* --58 - A - 45 - C ^* - <22 - X.

Mixture 3 has the composition: 10% MDW198, 20% MDW232, 0.9% ZLI4572, 69.1% H1. Mixture 3: C ^* pitch (22 ° C) = -0.19 μm; N ^* pitch = -36 μm; and the phase diagram is I - 82 - N ^* --55 - C ^* - <22 - X

FELIX008 is a commercially available FLC mixture with the phase diagram: I - 86 - N ^* - 75 - A - 70 - C ^* - (- 7) - X, and FELIX008 has a polarization of -9.6 nC / cm ² .

Example 5

Comparison of the contrast of DHF cells

Two DHF cells (2.5 μm thick) were prepared using a commercially available DHF mixture (Hoffman La Roche 5679) and of the (above) mixture 1.

The transmissive mode 2.5 μm cells were prepared by coating the inner surfaces of the plates with ITO (Indium Tin Oxide) electrode layers. A nylon layer was spun onto the electrode plates and dried. The nylon layer was rubbed in one direction with a soft rayon cloth. The alignment layers were rubbed on the opposite plates in a parallel direction. The plates were provided with spacers and assembled so that the rubbing directions ran parallel and in the same direction. The FLC mixture was heated to 100 ° C and introduced into the cell by capillary action. The FLC mixture was slowly cooled in the cell at a rate of about 0.5 ° C / min until the smectic C ^* phase was reached had been. This treatment resulted in planar aligned cells. Both cells, one containing mixture 1 and one containing Roche 5679, were placed between crossed polarizers and an appropriate voltage was applied to the electrodes to alter the optical anisotropy of the cells. The texture of the cells and the contrast of the cells were examined under the same conditions.

Photomicrographs of the texture of the two cells are in the 2 , View A, Mixture 1, and View B shown. The texture of the cell with the mixture 1 is much smoother. The contrast of the cell with mixture 1 was significantly higher (158: 1) than that of the 5679 containing cell (25: 1).

Claims (21)

A ferroelectric liquid crystal material comprising a chiral dopant and a host compound, wherein the ferroelectric liquid crystal composition has a ferroelectric phase and a chiral nematic phase at temperatures above the ferroelectric phase, characterized characterized in that the pitch of the natural helix in the nematic phase at temperatures of at least about 1 ° C to 2 ° C above the N ^* transition point at least about 4 times greater than the pitch of the natural helix in the ferroelectric phase at about room temperature and the chiral dopant is a chiral non-racemic compound of the formula RAZ ₁ -BOM wherein A and B are independently selected from 1,4-phenylene, 1,4-phenylene in which one or two of the ring carbon atoms are replaced by nitrogen atoms, or 1,4-cyclohexylene, Z ₁ is a single bond, an O atom, a -CO-O- or an -O-CO- group, R is a group of 1 to 20 carbon atoms, in which one or more, non-adjacent carbon atoms a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group may be replaced, and OM is a 2,3-dihaloalkyl structural unit of the formula

wherein ^{* represents} an asymmetric carbon atom, X and Y are independently halogen and R 'is a group of 1 to about 20 carbon atoms in which one or more non-adjacent carbon atoms are represented by a double bond, an O Atom, an S atom or a Si (CH ₃ ) ₂ group, or an acyl group -OCO-R ", where R" is a group having 1 to 20 carbon atoms, in which or a plurality of non-adjacent carbon atoms may be replaced by a double bond, an O atom, an S atom or a Si (CH ₃ ) ₂ group. A ferroelectric liquid crystal composition according to claim 1, characterized in that the host compound is a smectic C or C ^* host. A ferroelectric liquid crystal composition according to claim 1, characterized by the phase sequence: I, N ^* , smectic orthogonal, smectic ferroelectric - with decreasing temperature. A ferroelectric liquid crystal composition according to claim 3 characterized by the phase sequence: I, N ^* , A ^* , C ^* - with decreasing temperature. Ferroelectric liquid crystal composition according to claim 1, characterized in that the pitch of the natural helix in the chiral nematic phase at least about 10 times greater than the pitch of the natural Helix in the ferroelectric phase. A ferroelectric liquid crystal composition according to claim 1, characterized in that the ferroelectric phase is a smectic C ^* phase. Ferroelectric liquid crystal composition according to claim 1, characterized in that it further comprises a compensation means for the pitch contains the nematic phase, the same sign of the pitch in the ferroelectric Phase as the chiral dopant and has the opposite Sign of the pitch in the nematic phase as the chiral dopant. A ferroelectric liquid crystal composition according to claim 1, characterized in that the pitch of the natural helix of the composition in the ferroelectric phase is 0.25 μm or smaller at room temperature and the pitch of the natural helix of the nematic phase composition is at least about 1 ° C to 2 ° C above the N ^* transition point is 8 μm or longer. Ferroelectric liquid crystal material according to claim 1, characterized in that it is about 30 wt .-% or less of the chiral dopant. A ferroelectric liquid crystal material according to claim 1, characterized in that the dopant is a chiral nonracemic 2,3-difluoroalkyloxy compound of the formula

wherein ^{* represents} an asymmetric carbon atom, one of A or B is 1,4-phenylene and the other of A or B is 2,5-phenylpyrimidine and R 'is an alkyl group of 1 to 20 carbon atoms or an acyl group -OCO-R "having 1 to 20 carbon atoms. Ferroelectric liquid crystal material according to claim 10, characterized in that the configuration of the dopant 2-R, 3-R or 2-S, 3-S. A ferroelectric liquid crystal composition according to claim 10, characterized in that R 'is an alkyl group having 1 to 20 carbon atoms. A ferroelectric liquid crystal composition according to claim 1, characterized in that it comprises a host material containing at least about 10% by weight of one or more compounds of the formula R ₁ -OCD-OR ₂ in which one of the groups C or D is a 1,4-phenylene and the other of the groups C or D is a 1,4-phenylene in which one or more of the carbon atoms of the ring are each represented by a nitrogen atom and R ₁ and R _{2 are} independently alkyl or alkenyl of 1 to 20 carbon atoms or an alkylsilyl group of 4 to 22 carbon atoms. Ferroelectric liquid crystal composition according to claim 1, characterized in that it further comprises a compensation means for the pitch the nematic phase. A ferroelectric liquid crystal composition according to claim 1, characterized in that it further comprises one or more compounds of the formula R ₃ -EFZ ₂ -R ₄ wherein R ₃ and R _{4 are} independently alkyl or alkenyl groups of 1 to about 20 carbon atoms or an alkylsilyl group of about 4 to about 22 carbon atoms and wherein one of the groups E or F is 1,4 Is phenylene and the other of the groups E or F is 1,4-phenylene, wherein one or two of the ring carbon atoms are replaced by a nitrogen atom, and Z _{2 is} an oxygen atom, a -CO-O- or an -O-CO- group. Ferroelectric liquid crystal composition according to claim 15, characterized in that one of the groups E or F 1,4-phenylene and the other of groups E or F is 2,4-pyrimidine is. A ferroelectric liquid crystal composition according to claim 15, characterized in that R ₃ and R _{4 are} independently alkyl or alkenyl groups having 1 to 20 carbon atoms. A ferroelectric liquid crystal composition according to claim 1, characterized in that it further comprises one or more compounds of the formula R ₅ -GHZ ₃ -Cyc-R ₆ in which R ₅ and R ₆ independently of one another are alkyl or alkenyl groups having 1 to 20 carbon atoms or an alkylsilyl group having about 4 to about 22 carbon atoms, one of the groups G or H 1,4- Phenylene and the other of the groups G or H is 1,4-phenylene, wherein one or two of the ring carbon atoms are replaced by a nitrogen atom, Z _{3 is} a single bond, an oxygen atom, an -O Is -CO- or a -CO-O- group and Cyc is a 1,4-cyclohexyl group. Ferroelectric liquid crystal composition according to claim 18, characterized in that Cyc is a trans-1,4-cyclohexyl group is. Ferroelectric liquid crystal composition according to claim 18, characterized in that one of the groups G or H 1,4-phenylene and the other of groups G or H is 2,5-pyrimidine is. Ferroelectric liquid crystal composition according to claim 1, characterized in that the chiral dopant in the composition in an amount of 30% by weight or less is present, the host compound in the composition in an amount of 50% by weight or more and the pitch compensation agent in the composition in a concentration of 1 wt .-% or less is present. A ferroelectric liquid crystal composition according to claim 1, characterized by having an inclination angle greater than about 18 ° and a spontaneous polarization of greater than about 10 nC / cm ² .